In this Nov. 9, 2012 photo, Jason Derkosh holds his wife, Elizabeth, at the funeral of their 2-year-old son, Maddox, at Saint Bernard Church in Mount Lebanon, Pa. The Pittsburgh Zoo & PPG Aquarium says Elizabeth Derkosh, the mother of 2-year-old Maddox Derkosh, who was fatally mauled when he fell into a wild African dogs exhibit, is to blame for her son's death and shouldn't be allowed to sue. The zoo's attorneys made that argument in a response filed Monday Sept. 9, 2013 to the wrongful-death lawsuit brought by Jason and Elizabeth Derkosh. Maddox Derkosh died Nov. 4, 2012 after falling over a 4-foot-tall wooden railing into the exhibit when his mother lifted him up to get a better look.(AP Photo/Tribune Review, Justin Merriman)

This Nov. 5, 2012 file photo shows the overlook platform where a boy fell into the exhibit that was home to a pack of African painted dogs Sunday, Nov. 4, 2012 at the Pittsburgh Zoo and PPG Aquarium in Pittsburgh. The zoo's attorneys made the argument that the mother of the 2-year-old boy who was fatally mauled when he fell into the exhibit is to blame for her son's death and shouldn't be allowed to sue. The claim was in court papers written Monday, Sept. 9, 2013 in a response to the wrongful-death lawsuit brought by Jason and Elizabeth Derkosh, whose son, Maddox, died after falling over a 4-foot-tall wooden railing into the exhibit when his mother lifted him up to get a better look. (AP Photo/Keith Srakocic, File)

PITTSBURGH (AP) — The Pittsburgh Zoo & PPG Aquarium says the mother of a 2-year-old boy who was fatally mauled when he fell into a wild African dogs exhibit is to blame for her son's death and shouldn't be allowed to sue.

The zoo's attorneys made that argument in a response filed this week to the wrongful-death lawsuit brought by Jason and Elizabeth Derkosh, whose son, Maddox, died Nov. 4 after falling over a 4-foot-tall wooden railing into the exhibit when his mother lifted him up to get a better look.

"The injuries and damages sustained by Maddox Derkosh, including Maddox Derkosh's death, were caused solely by the carelessness, negligence, and/or recklessness of Elizabeth Derkosh," the zoo's attorney wrote Monday in the court filing. She "knew or should have known he could fall into the exhibit" and failed "to maintain a proper grasp of Maddox Derkosh after lifting him over the railing."

The parents' attorney, Robert Mongeluzzi, said the zoo "failed miserably in their solemn responsibility to prevent the attack" and has "now shamelessly attacked Maddox's grieving mother. We look forward to exposing their reckless conduct, in discovery, and at trial."

Allegheny County District Attorney Stephen A. Zappala Jr. investigated the boy's death and deemed it a "tragic accident," deciding not to prosecute his mother or zoo officials on endangerment or other charges.

But the standard of proof for criminal charges is greater than that needed to prove negligence or recklessness in a civil lawsuit, in which a jury could eventually decide that both sides share some responsibility.

"The fact that a horrible and tragic death of a child occurred at the zoo does not mean necessarily that the death was completely or even partially the zoo's fault," University of Pittsburgh law professor John Burkoff said Thursday. "The fact that the court of public opinion may have ruled already against the zoo is, and should be, irrelevant to the legal process."

Zappala determined the boy had vision problems and wore glasses, and that's partly why his mother lifted him to see down into the dogs' sunken enclosure.

Maddox became the only visitor in the zoo's 116-year history to die when he unexpectedly lunged out of his mother's grasp, over the wooden railing, and into a net meant to catch falling debris and trash. He bounced from the net into the dogs' enclosure about 10 feet below.

The animals immediately attacked the boy, who bled to death from massive injuries.

The zoo's 20-page response takes issue with claims that the exhibit wasn't safe, noting the wooden plank at the top of the railing was slanted 45 degrees toward the viewing platform "in order to keep visitors, including children, from sitting or standing on the railing."

Mongeluzzi has argued in the lawsuit that zoo officials were told by at least two employees that parents routinely lifted children onto the railing and, therefore, should have taken even more precautions.

"Maddox Derkosh was fatally attacked and killed at the zoo, when the zoo, after being warned by their own employees of the very accident that would occur, summarily dismissed their warnings," Mongeluzzi said Thursday.

Zoo spokeswoman Tracy Gray declined to comment on lawsuit but said in a statement that the zoo has met safety standards of the Association of Zoos & Aquariums and that the U.S. Department of Agriculture found no problems with the exhibit in 35 inspections since 2006.

The dogs have been relocated to other zoos and the exhibit now houses cheetahs.

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